A wireless communication system includes a number of base stations, scattered over a geographic area, to provide service for a number of communication devices, such as, personal data assistants (PDA), cellular telephones, messaging devices, mobile devices, and other suitable devices that can move throughout a geographic area. In a code division multiple access (CDMA) system, each base station continuously transmits a pilot channel signal on the forward channel. The pilot channel signal transmitted by each base station has the same spreading code but with a different code phase offset. The phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. The communication device monitors the pilots and measures the received energy of the pilots.
While the communication device is communicating with the base station(s), the communication device must constantly monitor and maintain three sets of pilot signals (collectively referred to as a Pilot Set): an active set, a candidate set, and a neighbor set. The active set consists of pilot signals associated with the forward traffic channels assigned to service the wireless device. The candidate set consists of pilots that are not currently in the active set but have been received by a particular communication device with sufficient signal strength to indicate that the associated forward traffic channels could be successfully demodulated. The neighbor set consists of pilot signals that are not currently in the active set or candidate set but are likely candidates for hand off. The remaining set consists of all pilots not found in the active set, candidate set, or neighbor set.
As the communication device moves from the region covered by one base station to another, the communication device promotes certain pilots from the neighbor or remaining set to the candidate set, and certain pilots of the candidate set are subsequently promoted to the active set by the base station. The communication device provides the base station or base stations pilot signal strength measurement data corresponding to the received energy of the pilot signals via a pilot strength measurement message (PSMM). In response, the base station notifies the communication device of the promotion from the candidate set to the active set via a hand off direction message. The process of continually updating the pilot set during a communication call is referred to as “pilot set maintenance.”
Using current technology, a problem with pilot set maintenance can be observed. In some instances, the communication device erroneously reports two duplicate pilots to the network thinking that they are different pilots, one an already reported active pilot, and the second the same pilot whom the communication device mistakenly thinks is in the remaining pilot set. The root of the problem is based in the configuration of the network, and thus the communication device currently has no way of preventing this from happening.
The network, for example, may have configured a rather large search window size for the remaining set. The network's remaining set window size dictates to the communication device's searcher how wide a search to use in order to find remaining set pilots. The larger the size of the window the wider the search. When the remaining set window size is too large it is possible that active, candidate, and neighbor pilots fall within the search window for a remaining set pilot that is close, since remaining set pilots will be immediately adjacent to nonconsecutive active, candidate, and neighbor pilots.
Pilots in CDMA are identified by their position in pilot PN sequence offset index space, hereafter referred to as PN (pseudonoise) space. Each pilot PN sequence offset index corresponds to a unit of 64 PN chips of a Forward Pilot Channel, relative to the zero offset pilot PN sequence. Pilots that are close in number are close together in PN space. For example the pilot identified as PN=36 is 128 chips away from the pilot identified as PN=38. If the pilot PN=36 was in the active set, pilot PN=38 was in the remaining set, and the remaining set search window was +/− 160 chips then when the wireless communication device searched the remaining set pilot PN=38 it would really find the active pilot PN=36 within its search window.
Further, if the network provides the communication device with conflicting searcher window sizes and PN information, the communication device can currently mistakenly report the same pilot in multiple entries in the pilot strength measurement message for handoff purposes.
For example, the network may command the communication device to do a remaining set search on a window that's +/− 160 chips in width. The communication device, for this example, is active on PN=36 and has seen a base station with PN=38 in its past history. Because each PN is equivalent to 64 chips, the search window of +/− 160 chips is equivalent to a window of +/− 2.5 PNs (which places the PN=36 active set pilot within the search window for the PN=38 remaining set pilot). The communication device then reports up two pilots (PN=36,38) even though just one exists.
Poor configuration of the remaining set (rset) window network parameter can cause poor voice quality for voice over IP (VOIP) data calls even in strong signal conditions. This is due to the excess communication between the communication device and the network for pilot set maintenance.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.